He said: “I have fished the Firth of Forth since I was a boy, from shore and boat, and never seen a fish anywhere near this size.

“But bluefin tuna have been spotted in the North Sea a bit more this year.

“They used to be seen regularly until herring shoals were fished out around 50 years ago.

“Hopefully increasing mackerel numbers are bringing them back.”

Forth Rivers Trust and Forth Marine Mammal Project caused a stir on social media when they shared pictures of the tuna sent in by Mr Hynd and Mr Rowan.

Jonathan Louis, operations and development manager for the Forth Rivers Trust, said: “When one of our followers sent us the photo we were all surprised in the office as we have never heard of Atlantic bluefin tuna being present within the Forth.

“We also never realised by posting it on our Facebook page how viral the post might go, with just over 50,000 views.

“It’s sad this one has washed up dead but hopefully this is a sign that they are making a return as there have been a number of sightings in the North Sea and off the west coast of Scotland.

“It would be fantastic to think this magnificent fish is making a comeback.”

Atlantic bluefin tuna are some of the fastest and most fearsome predators in the sea, swimming at speeds of up to nearly 50mph.

Growing to a gigantic 1,200lb, they are one of the most sought-after game fish on the planet, second only perhaps to blue marlin.

The species was once a more common sight in Scottish waters, but had pretty much vanished by the 1960s.

However, there has been an increasing number of sightings around these shores in the past few years.

Last year a bluefin tuna caused havoc when it got inside a salmon cage at a fish farm off Colonsay.

A giant specimen was hooked off the Western Isles in 2015.

Scientists at Marine Scotland have launched a project to find out more about the behaviour of the migratory species in Scottish waters, with individuals tagged in St Kilda being tracked as far away as the Azores and Bay of Biscay.

Anglers hope the increasing numbers of fish being seen in UK waters could see the start of recreational catch-and-release fishing, which has potential to be a lucrative business.

Most catches of Atlantic bluefin tuna are taken from the Mediterranean Sea.